Fig. 2

Thermal conductivity measurements of hexagonal boron nitride (h-BN) crystals. a Schematic of the nano-second transient thermoreflectance (TTR) technique. b Monitored thermoreflectance transients for the monoisotopic ¹⁰B h-BN sample (black solid line), normalized to the peak reflectivity modulation. Out-of-plane thermal conductivity (k_z), in-plane thermal conductivity (k_z) and the thermal boundary resistance between the Au transducer and h-BN (TBR_eff), were determined simultaneously via fitting the monitored TTR transients with the analytical thermal transport model. The error bars after the determined values correspond to the uncertainty of results (see Methods for the details of fitting and uncertainty analysis). Best fitting curve (red solid line) is overlaid with the measured TTR transients. The dashed curves are ±25% bounds on the best fitted k_z and k_r values. c Sample zoom-up; the carbon tape supporting the h-BN sample is considered thermally insulating due to its low thermal conductivity. d Sensitivity analysis with respect to k_r (S_r), k_z (S_z), and TBR_eff (S_TBR) for h-BN (material parameters taken here are k_r = 400 W m⁻¹ K⁻¹, k_z = 4 W m⁻¹ K⁻¹, heat capacity C_p = 740 J kg⁻¹ K⁻¹, TBR_eff = 50 m² K GW⁻¹, which are the typical values of h-BN^4,42)